A contracting muscle generates an electrical field that can be sensed with electrodes placed on the skins surface. The resulting voltage is defined as a surface electromyographic (sEMG) signal. Applications for using sEMG signal measurement are diverse and can range from sports and ergonomic activities to clinical evaluation of patients, as well as neuromuscular research applications which investigate motor control. Analysis of the sEMG signal can provide valuable information about muscle performance not obtainable by other means. The sEMG signal amplitude may be representative of force generated by the muscle, which unlike externally applied force measuring devices, can be used to assess the individual force contributions from a group of muscles acting together on a joint. Multiple sEMG sensors placed on the limbs can monitor muscle activity levels and coordination during gait studies, or in neurological disorders such as Parkinson's disease. Changes in a frequency spectra of the sEMG signal resulting from localized muscle fatigue can be used to more objectively assess appropriate activity levels and durations of tasks in the workplace. Using sEMG signal detection and analysis may be a valuable asset when investigating human muscular performance in these types of applications.
Conventionally, the configuration of a sensor designed to detect sEMG signals includes two electrode contacts placed on the skin over the muscle and oriented in a direction parallel to the muscle fibers. A third “reference” contact may be located at an electrically inactive location on the body. Disposable sEMG sensors designed for clinical use may include two electrodes filled with skin impedance reducing electrolytic gel or formed from hydrophilic gel; one for each signal input placed singularly, or in pairs, mounted on a flexible non-conductive pad adhered to the skin over the muscle. In some sensors, the two signal and reference contacts are placed on the same insulating pad in the form of an equilateral triangle. The electrodes are attached by snaps or spring loaded clips and connected to remote electronic preamplifier circuitry via individual lead wires. The preferred recording configuration is the single differential configuration where the voltage at each signal input contact is measured with respect the third reference contact and subtracted using a differential pre-amplifier circuit.
In addition, encased sEMG sensor designs incorporating integrated preamplifier circuitry with signal and reference electrode contacts secured to the bottom eliminate the need for individual lead wires and associated snaps or spring loaded clips. The sEMG signal output of these designs may be tethered to data acquisition hardware using a thin cable or can be completely wireless.
Despite recent advancements in sensor designs, sensors may still retain limited functionality in demanding applications, such as when recording data in uncontrolled movement disorders, in many sports activities, or in the work place environment. The widespread utilization of sEMG technology outside the laboratory environment has been limited by several factors related to the nature of the sEMG signal: The voltage amplitude of sEMG component of the signal detected by a sensor is inherently small, ranging from 10 microvolts to several millivolts. In addition to the sEMG signal component, the voltage at the sensor output includes the contribution from other noise sources generated by the inherent noise of the sensor's electronics, the electrolytic interface established between the metallic contacts of the sensor and intervening tissue, and artifact voltages induced from the movement of the sensor contacts with respect to the intervening tissue. Externally induced voltages from power lines and electrostatic (triboelectric) sources can also contaminate the detected sEMG signal. The magnitude of the contaminating electrostatic and movement artifact noise sources can be equal to or exceed the magnitude of the sEMG component of the sensors signal output when used in vigorous clinical, sports, and ergonomic activities, such as when monitoring neurologically impaired patients with flailing limbs, athletes performing jumping and throwing movements, or workers exposed to sudden accelerations and impacts. The problems of movement artifact and sensitivity to electro-static fields are especially severe when the sensor is placed under clothing garments. Even normal activities such as walking can induce electro-static voltage artifacts as a result of walking on carpet or contact with certain fabrics under low humidity conditions. These artifacts could easily be misinterpreted as muscle activity. Current sensor technologies restrict the user to conventional laboratory assessments; e.g. during sustained or repeated isometric contractions or during non-demanding dynamic tasks that are encountered in daily life, such as reaching for an object, walking/climbing stairs, lifting an object, or doing other non-vigorous, low-velocity activities. There may be additional burden to the subject and researcher on occasions when the subject is asked to change into a t-shirt and/or shorts prior to being instrumented so that no obstructions to the sensor from clothing are encountered.
All of the aforementioned tethered and wireless sensor configurations offer only a limited set of solutions for detecting high fidelity sEMG signals in applications involving dynamic contractions. The susceptibility of the described sensors to induced movement artifacts precludes use in vigorous applications and during conditions where electro-static fields may be generated such as sensor placement under an individual's clothing. The common practice of using adhesive tapes and bandages to secure the sensor to the skin can exacerbate the generation of electrostatic charge when in contact with garments.
It would be an improvement to provide a disposable sensor overlay covering configuration that secures and shields the sensor from mechanical and electrostatic disturbances when used under clothing.